Cross-sectoral impacts of the 2018–2019 Central European drought and climate resilience in the German part of the Elbe River basin

The 2018–2019 Central European drought was probably the most extreme in Germany since the early sixteenth century. We assess the multiple consequences of the drought for natural systems, the economy and human health in the German part of the Elbe River basin, an area of 97,175 km2 including the cities of Berlin and Hamburg and contributing about 18% to the German GDP. We employ meteorological, hydrological and socio-economic data to build a comprehensive picture of the drought severity, its multiple effects and cross-sectoral consequences in the basin. Time series of different drought indices illustrate the severity of the 2018–2019 drought and how it progressed from meteorological water deficits via soil water depletion towards low groundwater levels and river runoff, and losses in vegetation productivity. The event resulted in severe production losses in agriculture (minus 20–40% for staple crops) and forestry (especially through forced logging of damaged wood: 25.1 million tons in 2018–2020 compared to only 3.4 million tons in 2015–2017), while other economic sectors remained largely unaffected. However, there is no guarantee that this socio-economic stability will be sustained in future drought events; this is discussed in the light of 2022, another dry year holding the potential for a compound crisis. Given the increased probability for more intense and long-lasting droughts in most parts of Europe, this example of actual cross-sectoral drought impacts will be relevant for drought awareness and preparation planning in other regions. Supplementary Information The online version contains supplementary material available at 10.1007/s10113-023-02032-3.

With the exception of the the Ore Mountains of late Paleozoic origin surface geology of the GEB domain is characterized by deep sediments ordered by a sequence of glacial series. In the southern and western parts these sediments are topped by loam and loess layers while the central part including most of the Havel area is largely characterized by poor, sand dominated soils. Soils with higher water storage capacity can be found again in the loamy moraine regions of the most recent (Weichsel) glaciation at the north-eastern edge of the GEB or in the silty marshlands around the Elbe estuary.
Land use is dominated by non-irrigated agriculture (43 %), pastures (15 %) and coniferous forests (21 %). Such managed ecosystems -which are, especially on the sand-dominated soils, strongly affected by drought events -cover more than 85 % of the region (Copernicus 2020;see Fig. S1 and Table S1 for details). Built-up areas account for about 8 % of the land demand, and their share rises slowly but continuosly at the expense of agricultural areas, a trend expected to sustain at least in the vicinity of the big cities (Hoymann et al. 2016;Maretzke et al. 2021).   (Copernicus 2020 According to the German Weather Service (DWD-CDC, 2022a,b,c) the 1991-2020 averages of the air temperature at 2 m above ground were 9.4°C in the GEB and 9.7°C in the Havel area. There were upward trends over the last decades, a linear approximation since 1961 yields a regional warming rate of 0.35 K per decade (Fig. 2a). Precipitation varied about 656 mm per year (586 mm/a in the Havel area) and did not expose a significant trend, but the drought years 1976, 2003 and 2018 are clearly visible in Fig. 2b. Sunshine durations however increased with the temperatures (Fig. 2c): The trend lines indicate a surplus of 39.3 hours per decade. The 1991-2020 averages amounted to 1682 sunshine hours per year in the GEB and 1738 h/a in the Havel area. Figure S2: Annual domain averages of air temperature, precipitation, and sunshine hours. Extracted from monthly grid data (DWD-CDC 2022a,b,c) Since the 1970s the average air temperatures increased at about the double rate compared to the global average (Gulev et al. 2021). The notable increase in sunshine can at least partly be attributed to anthropogenic brightening (Wild 2014;Wild et al. 2021). The Havel subdomain is a little warmer, drier, and sunnier than the GEB as a whole owing to the lower average elevation (82 m less) and a more continentally located centre of gravity. This effect can also be seen in the isohyet map in Fig. S3: Besides the mountains, the coastal region near the Elbe estuary receives above-average precipitation. The map also shows the "rain shadow" of the Harz mountains, this area with spots receiving less than 500 mm in an average year is also the driest region of Germany.  Regarding the seasonality in the 1991-2020 period (Walter-Lieth climogram in Fig. S4), the coldest month was January with 0.8°C, the warmest July with 18.7°C, which means an average annual amplitude of 17.9 K. The driest month was April with 35.2 mm of precipitation, and the wettest July with 79.7 mm. A substantial share of the summer precipitation is generated in convective clouds; thunderstorms, hail, and heavy rain events occur most frequently during the hot season.
According to the Köppen-Geiger climate classification (as used by Kottek et al. 2006) the GEB largely falls into the Cfb zone: warm temperate, fully humid, with warm summers (warmest month between 10°C and 22°C); only the highest elevations of Harz and Ore Mountains have average January temperatures below −3°C and therefore belong to the respective snow climate Dfb. The Köppen-Geiger classification as "fully humid" is rather simple: The driest summer month is required to have more than 40 mm precipitation on average, and there must not be a strong imbalance of summer exceeding winter precipitation.
A more sophisticated aridity index (AI) was presented with the UNEP World Atlas of Desertification in 1992, now available in its third edition (Cherlet et al. 2018): AI = (Σ 30 (P i / ETp i )) / 30. In plain text, the index is the average of the 30 annual fractions of precipitation divided by potential evaporation within a climate normal period. Using gridded AMBAV (Penman-Monteith) calculations of potential grassland evapotranspiration (DWD-CDC 2022d), the Elbe region had an AI of 1.06 in the period 1991-2020, and the Havel area AI reached 0.90; both values are considered humid (0.65 < AI < 0.5 is considered subhumid, semi-arid is defined by 0.2 ≤ AI < 0.5, and AI values below 0.2 are arid with AI < 0.05 classified as hyper-arid).

S1.3 Population
Assuming homogeneous population densities within each of the 2529 municipalities overlapping the GEB, there were 18.143 million residents in the domain on 31 December 2020 (5.842 M in the Havel area). Especially Berlin attracted many people during the 2010s from in-and outside Germany: Between the 2011 census and the end of 2020 the German capital grew from 3.292 to 3.664 million inhabitants, thus contributing 71 % to the population growth in the GEB in this decade (Statistische Ämter 2014;DESTATIS 2021;BKG 2021).
Looking back into the 1990s and the first decade of the 21st century there was an exodus of younger people from the more rural regions of Eastern Germany who saw their employment perspectives deteriorate after the old industries of the former socialist economy had been closed down. Between 1994 and 2017, most districts in Eastern Germany lost 10-25% of their population while similar relative increases concentrated around the big cities of Hamburg and Berlin. Albeit the net out migration has meanwhile come to a halt, further population losses from the rural areas are expected until 2040 (Maretzke et al. 2021).
This can be explained by the age structure ( Fig. S5): The fertility among the population remaining in this part of the country is unsustainably low for decades, at about one child per woman in the 1990s, now stabilizing at 1.6 children (DESTATIS 2019a). The recent immigration of mostly younger refugees hardly altered the picture: outside the big cities with up to 4 % refugee share regularly only 1.0-1.5 % of the population is backed up by refugees. Consequently there are now about two people in the age bracket of 50-70 years per child or young adult up to the age of 20 living in Eastern Germany, and Maretzke et al. (2021) expect an aged population in the rural parts still in the year 2040 with averages above 50 years.

S1.4 Economy
The economy in the GEB has undergone a process of deindustrialization since the German reunification. With the exceptions of the metropolitan regions of Hamburg, the international port city, and Berlin, the federal capital, the GEB comprises the poorest regions of Germany measured by average available income and personal net worth of the inhabitants. However, a 25 % share of the secondary sector in gross value added (GVA) and the general prosperity of the region are still comparable to EU averages. Table S2 summarizes the situation; the year 2019 was chosen for reference because 2020 data were affected by the onset of the global pandemic. S2.3 Calculation of the Standardized Streamflow Index (SSI)

Figure S9: Stream runoff gauge catchments utilized for SSI computations. GEB is approximated by the Neu Darchau minus the Dresden catchment (area with elevation colours), and the Havel area is represented by the Rathenow catchment (hatched).
Like the meteorological drought indices SPI and SPEI the SSI compares monthly runoff values to distributions of other runoff records of the same month and provides the results in a normalized way as z-values. At the core of the Best Monthly Fit (BMF) method is the selection of a probability distribution from a candidate set to optimally approximate the data distribution for each calendar month separately. We used the candidate distributions proposed by Vicente-Serrano et al. (2012) (GEV, PearsonIII, Generalized Pareto, Lognormal, Log-Logistic, and Weibull), but did not fit the distribution parameters through L-moments but maximum likelihood estimation. The selection of the distribution to apply was based on p-values calculated by the Kolmogorov-Smirnov-test. Table S3 lists distributions and p-values for the catchment areas and calendar months.
Interestingly, the distribution of the monthly streamflows generated from the catchment area between the Elbe gauges representing the GEB (Fig. S9) could be approximated best by either lognormal or log-logistic probability distributions while the Havel streamflow at Rathenow required a bunch of different distributions. The p-Values are generally lower for the Havel catchment, probably due to noise in the Elbe data remaining from incomplete filtering of the inflow signal from upstream Dresden.   While Figs S10 and S11 illustrate the seizable drought impact on plant production, the primary economic sector in general was affected more by background downtrends as shown in Fig. S12. S13 S3.2 Forestry

S3.3 Inland fisheries
A report of the situation of the German inland fisheries and aquaculture is ordered annually by the supreme fishery authorities of the federal states. The issue for 2019 (Brämick n.d.) counted 166 professional fishing and 290 aquaculture enterprises for the five-state aggregate. Catches from rivers and lakes most of which are located in Brandenburg amount to approximately 1200 t per year, and the aquaculture production, centred in Saxony, reached about 4250 t/year. Using rough estimates of 15.00 €/kg and 7.50 €/kg as average retail prices for wild and aquaculture-produced fish, respectively, yields a gross revenue estimate of about 50 million euros.
Due to the direct consequence of the lack of water on fisheries, this sector is directly endangered by drought conditions. The case of the Seddin Lake (Großer Seddiner See; 52°16.5′N, 13°02.0′E; approx. 30 km south-west of Berlin) whose fisherman had to close shop due to low water levels was however more likely caused by sub-optimal water management than by drought alone. With SN and BB producing 47 % of carp in Germany, carp farming is a traditional kind of aquaculture in the Elbe River basin (Edebohls et al. 2021). Carp ponds are shallow water bodies with water depths around one meter. The ponds are drained in an annual cycle in order to harvest the carps. The water bodies differ in terms of water provision. In SN 25 % of the ponds are fed only from precipitation in mostly small basins. Other ponds are fed by above ground streams or groundwater (Ballmann et al. 2017). Salmonids are produced in flowing water sites. They prefer cold water temperatures and high oxygen levels. The most produced salmonid in Germany is the rainbow trout. Brämick (n.d.) reports increased fish mortality for 2018 and 2019 owing to high temperatures causing lack of oxygen, and dried-out ponds limiting the production. In this case the fishes are usually harvested before complete losses would be inevitable. However, carps are a warmwater species tolerating low oxygen levels (Ballmann et al. 2017). Therefore sites which held a minimum amount of water and level of oxygen had low economic losses.
The  Table S5: Especially water or energy sensitive sectors of manufacturing (Auerswald & Vogt 2010) Economical (sub-)section Energy sensitive Water sensitive Food and tobacco processing X Paper, publishing, and print X Chemicals and chemical products X X Rubber and plastic products X Non-metallic mineral products (glass, ceramics, etc.) X X Metals and metal products (except machinery) X S15 S3.5 Mining and related water issues in the GEB

S3.4 Manufacturing
The millennial history of mining in the mountainous areas, mainly for non-ferrous and noble metals, after WWII also for uranium in the Ore mountains, ended with the 20th century. Still in operation are only a few open-cast mining areas for lignite, remains of two furthering regions (Mitteldeutsches Revier, Lausitzer Revier) that reached their production peak already in the 1980s. and are now characterized by remodelled landscapes: disused mining pits usually being transformed into lakes.
Water was practically always more a disturbance than an advantage for mining activities (with the exception of early mechanical hydropower applications, e.g. for hammer mills). The open-cast lignite mining requires constant groundwater withdrawals through electric pumps to keep the excavation areas dry. Consequently, the recent drought was no problem at all for mining as such.
Before the German Reunification in 1990 up to 30 m³/s of groundwater were extracted from the Lusatian mining area and released into the Spree river. Meanwhile, most former open-cast mining areas are being flooded and groundwater storage recovers which naturally means less streamflow contribution (Grünewald 2001(Grünewald , 2010Koch et al. 2005). Consequently the 1976 and 1989 droughts are less pronounced in the Havel SSI compared to the time series for the entire contribution area between the Elbe gauges, while the downward SSI peak of 2018 was more negative for the Havel subbasin. Other upstream areas of the GEB may expose streamflow modifications from reservoir operations, the reservoirs in the Harz and Ore mountains are however small and can neither alter the general runoff seasonality nor provide sustained runoff during drought.
The planned emergence of lakes and waterways in the former mining area landscapes was however hampered by the drought. A water management centre in Lusatia, the border region of Brandenburg and Saxony, has been installed to carefully balance the filling of the new lakes with the minimum runoff requirements of the Spree River crossing Berlin. Shifting the goal of fully restored groundwater storages and filled artificial lakes (currently the year 2090 is aimed for the final lake fill; Scholz & Totsche 2022) further into the future is less problematic than a stagnant Spree river with water quality issues amidst the German capital (which still could be avoided). S16 S3.6 Energy and Water supplies While the recent drought did not cause any relevant outages in industrial and drinking water supply, record low groundwater levels and the perspective of decreasing water balances and groundwater recharge under climate change pose the question whether the high reliability of the system can still be guaranteed in the future. The associations of engineers, utility companies and water suppliers (the latter usually being public corporations in Germany) did their homework analysing the situation (Simon et al. 2019, DVGW 2021 and drafted a demand charter (BDEW-DVGW-VKU 2021). The German association of cities and municipalities took the same line (DStGB 2022), and the federal government published the draft of a national water strategy (BMU 2021).
These proposals build on the compound power of numerous measures drawn from the engineer's space of possible solutions. Tap water security is definitely given a high priority, and large sums of public money and water fees will be invested over time, for instance in new wells, additional pipe connections, forest conversions, rainwater processing plants, or intelligent demand monitoring and charging systems. In the long term, the price of tap water (and probably also taxes) will consequently rise a lot, and that will be the economic drought burden -not directly connected to any particular drought event.
Tap water fees change between supply areas, often municipalities. In 2019, an average German household paid approximately € 242 per year for their water (DESTATIS 2020), state averages were highest in the Saarland (€ 310) and North-Rhine Westphalia (€ 298). Within the Elbe area, tap water was most expensive in Thuringia (€ 267) followed by Hamburg (€ 256) and Saxony (€ 246). Interestingly, the average annual household fee is only € 157 in Berlin where the per-capita water availability is obviously most limited.
In addition to these water fees private end-users have to pay for the sewage water removal, usually a similar amount, so the average total water fees are currently about 400-500 € per household and year. This may probably rise by 20-30 % in the 2020s to finance the investments necessary for continued supply security. A surplus water fee of 125 € per year and household could grant about 5 billion € annually for infrastructure investments (1 billion € per year for the GEB), but any discussion about utility costs is currently focused on energy, and it is impossible to give sound price projections in times of international tensions and generally accelerating inflation.

S3.7 Construction
No hard data were available about drought effects on construction productivity, therefore this segment is not explicitely mentioned in the main article. The freshwater use of the construction sector is relatively low, with 44 million m³ in 2016 only at 1.4 % of the private household demand of 3118 million m³ (DESTATIS 2019b), and with practically no tap water restrictions there were no water shortage problems.
Weather is nevertheless a bigger issue in construction; some outdoor works cannot be done in bad weather conditions, but sunny days with strong radiation and high temperatures limit the human productivity outdoors. A global study (ILO 2019) found construction among the most affected branches but quantified the climate-change related productivity losses in Western and Eastern Europe in small fractions of a percent; losses during hot summer days may be balanced by shorter bad weather breaks in winter (IMAA 2019).
A special factor to take into account for construction workers is the additional heat stress through the protective clothing. Other stresses, namely increased ozone levels and uv radiation, need also to be taken into account. For construction workers in Germany skin cancer is a recognised occupational disease.
One of the most comprehensive summaries about heat effects on outdoor workers has been published in the online magazine of the statutory accident insurance institution (Berufsgenossenschaft) for the German construction sector (Pohrt et al. 2021).
There are however non-monetary losses: Accidents and fatalities become more frequent under high temperatures. This was tragically demonstrated by the death toll for the construction of football stadiums for the 2022 FIFA world cup in Qatar, but the effect is also statistically evident in the United States (Pohrt et al. 2021) and thus very probably a neglected problem in Germany, too.
S3.8 Transport Unfortunately, the statistics about hauling on inland waterways are inconsistent with respect to units. Most objective are ton-kilometres (load times distance), but these numbers are not easily available for subregions of Germany (monthly values are scattered about monthly reports). We therefore show the ton-kilometres for all of Germany in Fig. S15 and load-based indices for the relevant reaches of the Elbe River in Fig. S16. Table S5 illustrates the relative irrelevance of inland navigation upstream the port of Hamburg.   (2022) 136 400 000 t S3.9 Heat-wave related peaks in air conditioning demand The market for air conditioning systems can serve as indicator for uncomfortable heat in offices and private homes. There is a highly detailed statistics about production and trade for EU countries maintained by Eurostat: PRODCOM (Eurostat 2021b). The relevant product group has got the PRODCOM code 28.25.12.20 -"Window or wall air conditioning systems, self-contained or splitsystems". Figure S17 shows Germany's international trade in this kind of air conditioning. Aside from that, it seems that the production of air conditioning sets in Germany gained momentum in the first years of the millenium, and there might be still an increasing trend in output while the average prices fell from about 500 €/unit ten years ago to about 300 €/unit recently. Hence air condition will probably become less extraordinary in the GEB in the near future.
S3.10 Heat-related performance losses in the service sector The economic effect of heat-related performance losses in office work is hardly quantifiable. Zhao et al. (2021) reviewed 30 scientific articles on heat-related labour productivity losses and identified four different methods to quantify the economic impacts. The results from different world regions varied significantly and are not transferrable to the Elbe region. A similar meta-analysis of 35 studies was provided by Porras-Salazar et al. (2021) and came up with the shattering conclusion: "We could not find a relationship between temperature and office work performance neither for the range of temperatures measured in most of the office buildings (20°C-30°C) or a wider range (18°C-34°C).
[...] The lack of relationships does not necessarily refute that temperature affects the performance of office work." While the latter statement seems self-evident, both meta studies show that a generally applicable formula for the relationship between temperature and productivity of office workers is not on the horizon yet.
These findings question the validity of any single study such as IMAA (2019) estimating 540 million to 2.4 billion euro heat stress-related productivity losses for the entire German economy. And indeed: IMAA (2019) cite Hübler & Klepper (2007) as source for the 3-12 % loss range their figures S21 are based on, who in turn give Bux (2006) as source. However, Bux (2006) also discusses inconsistent findings in the literature and mentions the 3-12 % range without context, just as example for the typical uncertainty!
The German weather service (DWD) uses the "Klima-Michel" model for the thermal comfort of a reference person walking outdoors with moderate speed (Jendritzky et al. 1979, Jendritzky 1990). It is based on heat balance calculations for the human body and considers not only air temperature but also humidity, wind speed and radiation. A "perceived temperature" was developed as related index; values above 20 degrees indicate heat stress, different intensity levels are defined (Staiger et al. 1997). New insights about biometeorology and the temperature regulation of the human body led to the development of an overhauled index, UTCI (Jendritzky et al. 2010, http://www.utci.org).
A comparably refined thermal comfort index for indoor conditions would probably be needed to reliably describe and parameterize the relationship between thermal environment and human productivity in offices. Air humidity is still a neglected factor in productivity research, but also air movements from ventilation and heat radiation sources may be influential for office environments. Furthermore, different tasks are affected differently by heat stress (Lan et al. 2009), hence we have to conclude that no reliable forward estimation of productivity losses under heat waves is possible yet.

S3.11 Defence
The only military stationed in Eastern Germany, and consequently in most of the GEB, is the German army (Bundeswehr), other NATO allies have to stay outside this region as stipulated in Article 5 of the reunification treaty, officially "Treaty on the Final Settlement with respect to Germany" (BGBl. 1990 II S. 1317). The Ministry of Defence in Berlin and the military headquarters near Potsdam are also located in the GEB. How the defence readiness condition is affected by drought belongs of course to the domain of state secrets, but there were some news regarding military operations and forest fires: In 2018, the army supported the fighting of a wildfire at Treuenbrietzen, about 80 km south-west of Berlin, with helicopters and tanks. Also in 2018, a fire broke out in Saxony during a maneuver but was successfully extinguished by the military fire fighters at the same day. Another wildfire burning more than 900 ha of forest started in 2019 on the former military training range Lübtheen (about 53°17′N, 11°09′E). Due to the site's contamination with ammunition (it had been used for shooting continuously during 1936-2013) army troops and regular firefighters could not operate from a short distance.

S3.12 Education
Schools responded to the extreme heat events during summer months of 2018 and 2019 with shortened lessons or cancellation of courses. In BB and ST changes in the timetables are regulated by temperature measurements in classrooms or on the schoolyard, whereas in SN and in TH respective decisions are made independent from pre-defined thresholds.
S3.13 Human health: UV radiation  S3.14 Arts, entertainment and recreation The extreme drought conditions increased the potential of wildfires during 2018 and 2019 and thereby forced music and art festivals to delay or cancellation. Traditional Easter fires were cancelled for the same reason in 2019. As a measure of fire prevention, entering and visiting forest areas at night was not allowed in the Saxon national park "Sächsische Schweiz" during spring and summer of 2019.
The drought events affected tourism in the GEB in multiple ways. Forms of tourism depending on water availability in the first order were most affected. For instance, boating trips could not take place on the Elbe River during the 2019 drought. In 2020 the drought had impacts on canoeing tourism: locks had to be kept closed in order to maintain water levels.
Camping tourism is generally furthered by sunny weather conditions. Statistics of camping tourism in the five federal states ( The statistics also contain the numbers of foreign visitors to camping sites, but only SN and TH for each of the years 2015-2020. In Saxony, the share of overnight stays spent by foreign visitors decreased from 13 % in 2015 to 11 % in 2019, and then dropped to less than 4 % in 2020. In Thuringia, a stable foreigner share of 7 % was observed in the years 2015-2017; this decreased to 6 % in 2019, and finally dropped to 3 % in 2020. The increased share of local tourists at campsites during 2018 and 2019 suggests more spontaneous camping trips attracted by the sunny weather conditions. The disappearance of foreign visitors in 2020 was however a result of legal travel restrictions motivated by the pandemic situation.
Swimming pool operators clearly benefitted from the warm and dry swimming seasons; several visitor records have been thrown during the warm summer months of 2018.